When the owners spotted fleas on their two cats, they put “just a drop” of topical flea treatment on each one. Within hours the cats were very sick, and one of them was convulsing. The family rushed them to the nearest veterinary clinic, but both cats died.

The family had used flea treatment that was clearly labeled for use on dogs, but they told the clinic staff that they assumed a small amount would be OK for cats.

Astoundingly, a total of four cats died in a recent four-week period when their owners used canine topical flea treatment, those products some people call “spot-on” treatments. Just a dab, usually between the shoulder blades, quickly kills fleas on dogs. But those products can be deadly when people don’t read or precisely follow directions.

“I am very upset that the warning on the canine flea topical — ‘Do not use on cats’ — is so very small. I wish it said: ‘This Product Could Kill Your Cat’ in very large letters,” said Leslie C. Marino, practice manager at Greentree Animal Clinic in Pittsburgh.

None of the four cats was a regular client, but all were rushed there because the clinic was closer than the owners’ usual veterinary offices. The staff asked me to share this information with readers to prevent future tragedies.

I’ve heard that using dog products on cats can be dangerous, but this is the first time a veterinary clinic has provided real, heartbreaking anecdotes. Greentree Animal Clinic will send a copy of this column to representatives who sell veterinary drugs, Marino said.

Flea season has extended well into December and January in recent years, so everyone should be careful. Here are some other tips I’ve gleaned over the years from veterinary poison-control centers:

• Read the fine print on all medications and use a magnifying glass, if needed.

• Save the packaging for all medications, so if there is an adverse reaction a veterinarian will know exactly what you gave to your pet.